Consider depending on `unicode_script` crate

[dcsommer]

Script and ScriptExtensionsSet in icu4x is quite similar to Script and ScriptExtension in unicode-script. Is there a reason not to de-duplicate and have icu4x depend on unicode-script, which also aims to be a minimal implementation of UAX #24? unicode_script has a more efficient representation of a set of scripts, using bitflags. I'm not sure which is more efficient from a binary size perspective, however, which is also important. cc @Manishearth who appears to work on both of these projects :)

[Manishearth]

ICU4X is trying to minimize dependencies.

unicode-script was written for rustc; if ICU4X had existed then I wouldn't have created the crate.

I don't think ICU4X and unicode-script can share implementations for scx since ICU4X is doing zero-copy deserialization from pluggable data and would need to precommit to a ceiling on the number of scripts.